After some googling (did you know a dollar bill is approx 6.25"x2.75"?) and futzing with some measurements and sentiments, I came up with this (closed):
I was pretty pleased with how everything turned out, so I thought I'd attempt a custom envelope to go with the custom card. I pulled out the handy MS score-board, and after a few attempts, got a pretty nice envelope (white shimmer paper) to go with the card. I stuck some double sided tape on a scrap of the silver paper, and punched a heart out of it, and stuck it to the top of the envelope flap. When the waxy paper comes off the tape, it will be a perfect little closure for the envelope... see? Thoughts on the finished product?